There follows an explanation of this dual problem of protecting a structural core against damage from the environment and of revealing that damage if any, in the context of transport vehicles.
In this field, regulatory requirements laid down by certification organizations, in particular in aviation, specify that the dimensioning of vehicle structures must take account of the effects that the environment can have on their structural integrity, their ability to withstand mechanical impacts, and the potential for propagation of damage, particularly for parts that are strongly subjected to fatigue stresses, i.e. parts under the combined action of both static and dynamic loading.
It is in this context that structural assemblies made of composite materials were introduced.
Such structural assemblies of composite materials thus make it possible to reduce the mass of a vehicle, to avoid problems due to corrosion, and also to provide positive responses to the requirements of certification organizations.
In practice, structural assemblies made of composite materials have given satisfaction in many aspects.
In particular their mechanical performance (stiffness, strength) have been found to be at least equivalent, and often better than the performance of assemblies made of isotropic metals (light alloys, titanium, steels, and other similar materials), while also achieving for equivalent structural assemblies, a substantial saving in weight.
These advantages come in particular from the intrinsic makeup of the materials used in such structural assemblies made of composite materials.
In general, prior art composite material structural assemblies comprise an organic matrix with continuous or discontinuous fiber reinforcement (e.g. textile reinforcement).
The organic matrix, e.g. a thermosetting resin, serves to keep the reinforcement in position and properly oriented.
The matrix serves to transfer loads between the parts of the reinforcement and thus acts as a binder in the composite material structural assembly.
The reinforcement is made in particular out of fibers (glass, carbon, aramid, . . . ). When the fibers are continuous, the reinforcement often has its fibers extending in the direction of the main mechanical stresses imparted to the composite material structural assembly.
Thus, the reinforcement confers on the composite material structural assembly its mechanical performance in terms of stiffness and breaking strength, while also achieving a saving in mass compared with equivalent structural assemblies made using metal alloys, for example.
There exist several conventional types of composite material structural assemblies.
One type of composite assembly is said to be “monolithic”, comprising a stack built up from a predetermined sequence of individual plies having reinforcement, e.g. in the form of fabrics or unidirectional sheets, and impregnated with an organic matrix.
By way of example, these individual plies are made up of carbon fibers impregnated with a thermosetting resin.
Another type of composite material structural assembly is the “sandwich” type.
A sandwich composite assembly has a core sheet of foam or honeycomb within a stack constituting a predetermined sequence of individual plies provided with reinforcement impregnated with an organic matrix.
Nevertheless, whatever their type, when they have at least two plies of impregnated textile reinforcement, structural composite assemblies present mechanical behavior that is strongly anisotropic and they thus present a specific mode of degradation in the face of external attack.
Because of their stratified composition, and because of the weak mechanical properties of the plies in directions that are transverse to the direction of the reinforcement, such composite structural assemblies are generally sensitive to shock or impact concentrated at a point.
Shocks or impacts cause delamination and loss of cohesion between the various plies of a composite structural assembly.
The delamination and loss of cohesion can lead to complete failure in the mechanical behavior of a composite structural assembly.
Mostly, when the delamination occurs within the composite assembly, it is not always identifiable by ordinary inspection techniques, in particular visual inspection. This is unlike metal structures, where any damage caused by an impact is generally detected visually.
Thus, unlike metal structures, the damage caused by an impact is not visually detectable on a composite structure.
Unfortunately, in numerous examples of composite structure assemblies, integrity of the assembly is essential for the safety of the intended vehicle and its passengers.
As a result, at present, it is necessary, at regular intervals, to perform inspection using ultrasound means in reflection or in transmission in order to be sure that a critical composite structural assembly has indeed conserved its mechanical performance.
It will be understood firstly that such inspection techniques are not completely reliable.
Secondly, they are difficult and often expensive to implement. In particular it is often necessary to take the vehicle temporarily out of service for inspection purposes, which is penalizing in operating terms, particularly in the field of aviation.
To sum up, composite structural assemblies are subjected in practice to damage because of the impacts and shocks they suffer, which can lead to situations that can become catastrophic insofar as the damage can be hidden and go unrepaired.
In order to mitigate this situation, one solution leads sometimes to providing composite structural assemblies that are overdimensioned, e.g. by adding reinforcing plies.
Such overdimensioning is particularly important when using unidirectional textile reinforcement, since such reinforcement is less tolerant to damage than is continuous woven reinforcement.
In addition, composite structural assemblies that are overdimensioned in this way lose some of the advantage associated with the light weight of composite materials, e.g. when compared with metals.
Another solution consists in making the reinforcement out of fibers that present high resistance to impact, such as aramid fiber.
Nevertheless, the highly hygroscopic behavior of such high-strength fibers leads to problems due to moisture being absorbed and degrading the interface between the fibers and the matrix.
Furthermore, such fibers are difficult to work and to cut, so they are particularly expensive and difficult to implement industrially.
In addition, in order to find a solution to the problem of enabling composite structural assemblies to withstand shocks, certain suppliers of textile reinforcement preimpregnated with an organic matrix, in particular of the epoxy type, have optimized the formulation of such matrices by incorporating thermoplastic plasticizers.
Unfortunately, matrices including thermoplastic plasticizers are expensive and they affect the mechanical performance of composite structural assemblies in ways unrelated to impact resistance, for example stiffness at high temperature.
In addition, those solutions do not solve the crucial problem of revealing damage.
In addition to impact, the intrinsic mechanical characteristic of composite structural assemblies can also be degraded by other kinds of attack from the environment.
Such other environmental attacks are constituted in particular by erosion (specifically under the effect of rain), grooving or nicking, lightning, flame, temperatures higher than acceptable values, and certain aggressive chemicals such as solvents.
On the same lines, composite structural assemblies can present vibratory behavior that is different from that of an equivalent structure made of metal, for example.
To counter such environmental attack, the problem of providing protection and of revealing damage does not have a prior art solution, any more than does the problem of impact damage.
Various documents are mentioned below that illustrate the above in prior art structures.
Document DE 4 208 842 describes an adhesive strip for protecting an edge of a helicopter rotor blade against erosion.
The strip is made of metal and it is stuck to the blade by means of a coating of metal particles.
That document does not describe revealing damage to the structure that is to be protected, since in the event of perforation the protection is removed and replaced.
Document DE 10 340 561 describes a structural member made of lightweight composite material for a motor vehicle or an aircraft such as a helicopter.
The member which is of plane or concave shape is designed to withstand crashes, impacts, or explosions.
That anticrash material is provided with one or more solid layers of metal (in particular aluminum) or of glass fibers.
An elastomer layer is placed on said layer, with the elastomer containing a tangle of reinforcing fibers.
Document EP 1 034 921 describes a multilayer composite structure for protecting members that are subjected to erosion, such as the lift members of aircraft.
In that structure, a covering layer is assembled on a substrate of material reinforced with metal fibers.
The covering layer comprises a thickness of metal fibers and a metal sheet that forms the outside face of the structure.
In order to improve the surface adhesion of the covering layer on the substrate, the porosity of the face of the thickness that is facing the metal sheet is less than the porosity of its face that is facing the reinforcing fibers.
Document EP 1 344 634 describes fabricating a helicopter rotor blade by inserting a core in a matrix that is then covered in layers of composite material, the resulting assembly being placed in a mold and then heated.
Document GB 2 242 002 describes a support for a heavy object such as a building that is to be protected against seismic shocks.
That support comprises alternating rigid plates and layers of rubber.
Those plates and layers are not bonded to one another, at least not in a peripheral zone.
Document WO 2004003403 describes a spring blade made up of a plurality of layers of different polymer materials.
A resilient inner layer acts as an absorber and defines a housing for receiving a filler insert.
None of those documents provides protection against environmental attack which:                firstly increases the intrinsic ability of the composite material to withstand one or more types of attack, e.g. by raising the energy level needed for the impact to cause damage; while simultaneously        leaving a permanent trace on the composite structure in the event of it being damaged, so as to make it easy to see that damage has been done.        